Behavioral and Lethal Effects of La Salt and a Mixture of Cu and La Salt on Daphnia magna Straus

The study of joint effects of REE and HM is relevant, since they are often satellite deposits, their areas of application are similar, and an increase in concentrations of elements of these groups in the areas that are not places of their extraction and enrichment is proven. The purpose of this work was to find out the pre-lethal and lethal effects of La, Cu and their equimolar mixtures in tests for Daphnia magna Straus. Bioassays of artificially polluted natural waters, initially free of toxic elements, was carried out. In bioassays on the mortality of D. magna in the space of 96 hours it was found that acute toxicity of copper sulfate solutions is observed at the calculated concentration of Cu2+ 0.1 mg/L (0.0016 mmol/L), and the acute toxicity of lanthanum sulfate is when the dose of La3+ is equal to 50 mg/L (0.36 mmol/L). In the solutions comprising mixtures of Cu and La salts (1:1 calculated using metals), the concentrations of which are equimolar to the investigated solutions of copper sulfate, the mortality of D. magna begins in the solution containing 10 times less toxic elements. It was found that 25% of individuals died in the variant “0.00016 mmol/L”, the mortality of 100% of individuals was at the total metal concentration of 0.0008 mmol/L. The solutions containing La (0.072–0.72 mmol/L) and Cu (0.00016–0.0016 mmol/L) naturally inhibit the motor activity of D. magna by 1.3–5.3 times and 1.2–1.9 times in 1 hour and 1.7–2.8 and 1.4–2.2 times in 24 hours, respectively. The solutions containing mixtures of Cu and La salts inhibited the motor activity of D. magna in the same way as copper sulfate solutions with the Cu2+ concentrations equimolar “Cu2+ + La3+”. Therefore, when testing the solutions with the same molar concentrations of Cu2+ and the mixture of “Cu2+ + La3+” it was shown that La potentiates the pre-lethal effect of Cu to the level of individual effects of Cu. The additions of La salt to the solutions containing pre-lethal doses of Cu lead to lethal effects of such mixtures for D. magna.